Machine vibration is a common byproduct of machine operation. For many machines there is an acceptable range of allowable vibration associated with proper machine functioning. For machines with rotatable parts, such as rotary engines for example, this is especially true. In order for machines and especially for rotary engines to function properly proper clearance between its various internal parts must be maintained. In various rotating engines, a rotor or rotating member is closely confined within an outer housing or casing. The distance between the rotor and casing is typically quite small. It is critical that the gap or distance between the casing and the rotating member, referred to as running clearance, be maintained within predetermined acceptable limits in order to ensure the uninterrupted, effective operation of the machine.
An example of such a rotary engine discussed above is a hot gas turbine engine. An exemplary hot gas turbine engine of the type described herein is an aircraft gas turbine engine. In such an engine, a turbine wheel or rotor having a circumferentially extending row of spaced apart blades extending therefrom is closely confined within a casing. The casing encircles the rotor to define a hot gas flow path in combination with the blades. The hot gas flows along the flowpath and impacts the blades causing the turbine wheel to rotate and this rotation in turn rotates the turbine wheel.
In the example of the gas turbine engine, preservation of a minimum clearance gap during engine operation is necessary to avoid rotational contact between the blades and the casing as the blades rotate within the casing. If the clearance is too small, the blades will contact the casing. Such contact could produce component part damage, engine failure and a loss of power. Conversely, if the clearance is allowed to become too large, the fuel consumption of the gas turbine will increase. Therefore it is necessary to closely monitor the running clearance of a turbine wheel during its operation.
Typical commercial aircraft engines use accelerometers and an associated signal conditioning system to monitor engine vibration, and these systems provide an indication of such vibration to the flight crew. Most current aircraft vibration monitoring systems locate the vibration monitoring computer in the electronics bay in the body of the aircraft while the accelerometers are located on the engine, and typically on the outer casing of the engine. Also, initial signal conditioning units, such as amplifiers, may be located on the engine, in the engine strut, or in the aircraft's electronics bay.
While accelerometers are commonly used to track and record engine vibration, their use is accompanied by several shortcomings. The accelerometer measures vibration at the engine casing rather than directly at the rotor. The accelerometers measure acceleration at the engine casing. The casing is a secondary source of vibration. It is more desirable to measure vibration at the primary vibration source of the rotary engine rather than at the secondary vibration source. Also, using the accelerometer is an additional system component that adds weight to the engine. Weight considerations are always of paramount importance for aircraft due to their direct negative affect on engine fuel consumption and efficiency.
What is needed is a system for more accurately tracking vibration where the system does not negatively affect the operating efficiency of the engine or other sensed device.